This invention relates to a method of making glow plugs and to glow plugs which are used to ignite fuel in internal combustion engines with an internal electrical resistant element which is enclosed within a sheath and which is exposed to the fuel within the internal combustion chamber.
The present invention relates to a glow plug which is used in a diesel engine typically for powering an automotive engine for igniting the fuel quickly, for example, in under ten seconds, and which is produced at sufficiently low cost to be commercially competitive with existing glow plugs. It is to be appreciated that the glow plug is subjected to rather hostile, environmental conditions within the cylinder wherein engine vibrations are present, the temperature at the plug is at least 1100 degrees C, and the hot combustion gases are under high pressures and are corrosive in nature.
The heating element sheath projects outwardly into the combustion chamber from an encircling housing or casing which is usually threaded at one end and threaded into the cylinder block. The projecting portion of the tubular sheath is usually secured in a gas tight manner by brazing to the housing by a filler tight brazing at the end of the housing encircling the sheath. The brazing provides a gas tight seal between the sheath and the internal bore of the casing so that the high pressure gas for example at 400 psi at ignition time will not move along the interface between the sheath and the casing bore wall and eventually penetrate into the interior of the sheath at the open opposite end of the sheath. Such brazing is shown in U.S. Pat. No. 3,749,980.
The tubular heating element disposed within the housing has a central electrode projecting from its interior end which needs to be electrically isolated from the casing and which also needs to be sealed in a gas tight manner with respect to the sheath wall to prevent the intrusion of air bearing oxygen into the interior of the heating element. U.S. Pat. No. 4,252,091 discloses providing a grooved bushing having matching grooves to fit into the electrode and into the grooves formed in an end of the tubular sheath to provide a sealed, tortuous passage against the penetration of air into the interior of the heating element and into contact with the magnesium oxide and the heating element coil. Additionally, this patent discloses that a filler material having a high affinity to oxygen such as aluminum or magnesium may be placed over the top of the heating element and the bushing and captured below an "O" ring to assist in providing a gas tight seal against air intrusion into the interior of the tubular heater element. The present invention eliminates the necessity for the brazing operation such as disclosed in the U.S. Pat. No. 3,749,980 and provides a more simple and inexpensive interconnection between the glow plug housing and sheath. The present invention uses a compressible, insulative bushing which is compressed during assembly to provide an effective, air-tight seal not only between the surrounding housing and the tubular sheath but also between the externally projecting electrode and the tubular sheath to provide against penetration of oxygen into the interior of the heating element.
In accordance with the further aspect of the present invention there is provided a new and improved fast-start heating composite coil which uses a nickel element to allow the initial voltage to provide a fast start and a Kanthal coil portion to provide the heating. The increasing resistance of the nickel limits the maximum temperature. Such a fast heating of the plug followed by a self-regulating characteristic avoids the high operating temperatures which would be ultimately produced without the regulating feature of the nickel wire coil and yet, provides for more instantaneous starts of engines in cold climates wherein the delayed startup time has been a particular problem.
The present invention has solved the problem of how to provide a series resistive network for bringing the sheath up to ignition temperature and then to plateau so as not to exceed a predetermined temperature for example about 2100 degrees F. after ninety seconds of operation. The present invention provides a practical and effective fast start glow plug which will meet the necessary and commercially desired criteria for starting automotive engines in cold climates.
A general object of the invention is to provide a new and improved glow plug and method of making the same as contrasted with the prior art glow plugs as above described. Another object of the invention is to provide a new and improved mechanical seal in a glow plug to prevent the intrusion of oxygen into the interior of the heating element.
A still further object of the invention is to provide a new and improved fast start glow plug.